Illuminator for inspecting substantially flat surfaces

ABSTRACT

Illuminator apparatus for illuminating a workpiece during visual testing thereof, the illuminator comprising: a source of illumination that illuminates a portion of the workpiece with on-axis illumination centered at a first angular direction and having a first intensity and with off-axis illumination having a second intensity; and an optical viewing system, that views said portion of the workpiece and accepts light reflected from the workpiece over a range of angular directions, centered at a second angular direction, said range of angular directions defining said on-axis illumination, wherein said first intensity and second intensity art separately adjustable and wherein the first angular direction is different from the second angular direction.

FIELD OF THE INVENTION

The invention relates to the illumination of substantially flat surfacesin order to perform visual inspection of the surfaces and in particularto the illumination of printed circuit boards in order to perform visualinspection thereof by machine.

BACKGROUND OF THE INVENTION

Modern printed circuit boards are typically laminated from numerouslayers, the planar surface of each layer comprising an intricate patternof conducting regions, formed for example from 1-2 mil thick copper,separated by regions of non-conducting substrate. A fault in anyintermediate layer of the board may result in malfunction of the entireboard. Consequently, it has become standard practice to check for theexistence, integrity and shape of features existing on each of theprinted circuit board layers during manufacturing and prior tolamination.

The inspection of complicated printed circuit board layers is generallydone optically by machine. The printed circuit board is placed on themachine to enable partial viewing of the board by the collecting opticsof an imaging system, and is subsequently scanned. While passing throughthe field of view of the collecting optics, it is illuminated by anappropriate illumination system.

In prior art systems a single CCD array and illuminator is typicallyemployed. Such conventional arrays and illuminators are typicallyinsufficiently long to acquire an image of the entire width of theprinted circuit board in a single pass. As a result, in addition tomoving the board and collecting optics relative to each other in aprincipal scanning direction, the machine must additionally move theboard and collecting optics in a second, orthogonal, direction in orderto construct an image of the entire board. The result is a composite,image comprised of long thin contiguous strips, on the order of 0.5 milwide, acquired sequentially from subsequent passes of the CCD array overdifferent sections of the printed circuit board surface. Each stripapproximates the field of view of the CCD array.

The acquired image is next analyzed and a map of features on the boardis prepared. This resulting map can then be compared, by computer, to astored map of predetermined features or design rules to which the boardis supposed to conform.

Different regions on a printed circuit board may be distinguished bytheir reflective behavior when exposed to a source of light. Forexample, the conducting material on a printed circuit board is generallya more specular, if somewhat diffusing reflector of white light relativeto the substrate material which is generally more diffuse. Moreover, byrelying on differences in spectral reflection properties, it is possibleto enhance the contrast between laminate and conductor by usingappropriate color filters.

Because image processing of a an image acquired from a printed circuitboard relies on an analysis of the reflective properties of its variousfeatures, the process can be highly sensitive to the qualities of thelight used to illuminate the board. For example, boards are made up ofvarious materials each having differing reflective properties.Additionally, the surface of boards have a topographical relief that maybe resultant both from the cross sectional shape of the conductors, aswell a surface microstructure. As a result, the intensity or brightnessof a reflection of an inspected feature on a board may be dependent notonly on the inherent reflective properties of its materials, but also onits surface topography.

To provide an effective illumination in automated optical inspectionapplications it is necessary to mitigate the effects of topographicalvariations on a board's surface. Thus, it is known to highly concentratelight along a relatively thin line by using a source configured toemanate light over a relatively wide solid angle of illumination.

It is believed that the following patents represent the state of the artin high intensity concentrated illumination for automated inspection ofprinted circuit boards: U.S. Pat. No. 4,421,410 to Karasaki et al, U.S.Pat. No. 4,877,326 to Chadwick et al, U.S. Pat. No. 4,801,810 to Koso,U.S. Pat. No. 5,058,982 to Katzir et al, and U.S. Pat. No. 5,153,668 toKatzir et al., the disclosures of all of which are incorporated hereinby reference.

In some conventional illuminators that provide a wide solid angleconcentrated illumination, the strip of the board being inspected isilluminated with light from three linear illumination sources that arefixed substantially parallel to the strip. Light from a first of theillumination sources is concentrated onto the strip from a directionsubstantially perpendicular to the surface of the board by a cylindricallens or a section of an elliptical cylindrical mirror running the lengthof the first light source. Light from a second illumination source isconcentrated by a similar lens or mirror onto the strip from a firstoblique angle with respect to the normal to the surface. Light from athird illumination source is concentrated similarly onto the strip froma second oblique angle to the normal that is equal and opposite to thefirst oblique angle. In some of the prior art illuminators, the threeillumination sources are configured to create a contiguous solid angleof concentrated light.

For the purposes of clarifying terminology as used herein, it is notedthat on-axis illumination is defined as illumination that a reflectingsurface parallel to the plane of the workpiece would specularly reflectin a direction along the axis of the collecting optics. Off-axisillumination is defined as illumination that is reflected into thecollecting optics by surfaces that are not parallel to the plane of theprinted circuit board. In the conventional illuminators, the on-axisillumination illuminates the board from a direction substantially normalto the area of the board being illuminated, while the off-axisilluminators each respectively illuminate the board from directions oneither side of the on-axis illumination.

The prior art concentrating broad solid angle illuminators comprise manyoptical components that must be accurately positioned in order toprovide a wide solid contiguous angle of illumination. Settings of thevarious light sources must also be accurately adjusted. Furthermorethese settings and positions must be stabilized and accuratelymaintained in an environment subject to vibration and large heattransfers. Additionally the “seams” or boundaries between the on-axisillumination and the two off-axis illumination regions are generallydefined by the edges of the mirrors or lenses used to concentrateon-axis and off-axis illumination on a board. These seams or boundariesare therefore sharp and generally obtrusive. This makes it difficult toassure that on-axis illumination and off-axis illumination are smoothlyblended to provide a substantially uniform illumination throughout thebroad angle of illumination over the area of an illuminated strip.

As a result of these difficulties, mechanical and optical components ofprior art concentrated illuminators require very tight tolerances andare relatively expensive. Furthermore these difficulties have restrictedthe lengths of the effective region of illumination to the order of 15cm, which length is often less than the width of the board beinginspected.

An illuminator for providing concentrated light, but having analtogether different design is shown in U.S. Pat. No. 4,801,810. In thispatent an elliptical reflector comprising approximately one-half of anelliptical cylinder is used to illuminate the surface of a printedcircuit board. The axis of the ellipse is placed at an oblique angle tothe surface of the board, with the surface being placed at one focus ofthe ellipse and a single source of illumination being placed at thesecond focus. An imaging system images the illuminated line on the boardfrom an angle equal (but opposite) to the angle at which it is directlyilluminated by the source. This system provides uneven off-axisillumination of the line on the board and does not allow for independentadjustment of on-axis and off-axis illumination since only a singlesource is used for illuminating the board from all directions.

SUMMARY OF THE INVENTION

The present invention is generally described in the context ofillumination and inspection of printed circuit boards or theirconstituent layers which comprise a metal pattern on a non-conductingsubstrate. However, as will become evident, the present invention isapplicable to the automated inspection of many other types of patternedsurfaces such as artwork, negative or positive masters (photomasks),hybrid circuits (with suitable scaling) and the like. To emphasize thisbroader applicability of the invention, the term “workpiece” is usedherein to refer to these broader applications and the term “printedcircuit board” is used when referring to printed circuit boards, properor their constituent layers.

One aspect of some preferred embodiments of the invention provideson-axis and off-axis illumination of workpieces from the same apparentlycontiguous source, wherein the illumination intensity of the on-axis andoff-axis illumination is separately adjustable.

Differently stated, this aspect of the invention provides for a seamlesswide angle source of concentrated on-axis and off-axis illumination,wherein the intensity of the illumination for each of the on-axis andoff-axis illumination is separately adjustable.

Prior art systems provide for either such adjustability or for aseamless illumination. As indicated above, such seamless illumination isdesirable to avoid artifacts in images of printed circuit boards.Separate variation of on-axis and off-axis illumination is desirable toallow for adjustment of the two separate illuminations to achieve auniform level of lighting, or to account for various reflectivities androughness of the objects being imaged. For example, when viewingphotomasks, in which black lines are formed on a clear substrate and thesubstrate is imaged against a matte surface, optimal contrast isachieved when the on-axis illumination is zero to avoid specularreflection from the “black” lines which reflect weakly, but specularly.Additionally, the signal to noise ratio of images of printed circuitboards may be optimized by reducing the intensity of off-axisillumination relative to the on-axis illumination. This is the result ofan increase in reflection from the non-conducting portions of the boardsand a decrease in mottling as off-axis illumination is increased.

An aspect of some preferred embodiments of the present inventionprovides for the center of the on-axis illumination to be at an obliqueangle to the surface of the workpiece. In addition, the angular extentof the off-axis illumination is preferably substantially equal on bothsides of the on-axis illumination.

Another aspect of some preferred embodiments of the present inventionalso provides for the center of the on-axis illumination to be at anoblique angle to the surface of the workpiece. In addition, this aspectprovides for the on-axis and off-axis illumination to be separatelyadjustable.

As known to the inventors, prior art systems which provide for thecenter of the on-axis illumination to be at an oblique angle to thesurface of printed circuit boards provide for neither separateadjustability nor equal angular extent of the off-axis illumination oneither side of the on-axis illumination.

Equal angular extent of off-axis illumination results in less mottle,while oblique illumination allows for a seamless transition between theon-axis and the off-axis illumination. However, the combination of thesequalities in a single illumination system has not been available in theprior art.

Furthermore, prior art systems with non-normal on-axis illuminationprovided illumination from substantially all directions above thesurface being illuminated (except for the direction of the imagingsystem). The present inventors have determined that it is desirable tolimit the extent of off-axis illumination, at least when viewing printedcircuit boards, since, while the mottle is decreased with increasingangle of illumination, the signal from the nonmetallic portions of theworkpiece increases faster than that from the metallic portion,resulting in a reduction in signal to noise ratio.

One aspect of some preferred embodiments of the present inventionprovides an illuminator for providing a wide solid angle of continuousconcentrated illumination, comprising fewer optical elements than manyprior art broad angle concentrating illuminators and/or less complicatedconstruction of the parts.

Another aspect of some preferred embodiments of the present inventionprovides for a smooth seamless off-axis illumination and seamlessblending of on-axis and off-axis illumination in an illuminatorproviding wide solid angle of continuous illumination, even when theintensities of the on-axis and off-axis sources are different.

Another aspect of some preferred embodiments of the present inventionprovides an illuminator that can provide wide solid angle ofconcentrated illumination for strips on the surface of a workpiece thatare longer than strips that are illuminated by prior wide solid angleconcentrating illuminators. This is made possible, in some measure, bythe simplification in construction of the parts used in the system andin their alignment.

According to another aspect of some preferred embodiments of the presentinvention, a method is provided for determining the position and size ofan optimum virtual illumination source to be used as a source ofillumination when the actual source of illumination is used with adiffuser.

According to still another aspect of some preferred embodiments of thepresent invention an automated optical inspection system is provided,wherein inspected workpieces are illuminated using an illuminatorconfigured according to the teachings herein.

According to still another aspect of some preferred embodiments of thepresent invention, a method for inspecting workpieces is provided, suchmethod incorporating the steps of illuminating the workpiece withillumination provided by an illuminator according to the teachingsherein.

According to an additional aspect of some preferred embodiments, asuitable reflector for use in the illuminator according to the teachingsherein, and method for manufacture thereof, is provided.

An illuminator in accordance with a preferred embodiment of the presentinvention comprises a single linear light source hereafter referred toas an “on-axis source” to provide on-axis illumination. A differentsingle linear light source, hereafter referred to as an “off-axissource”, provides off-axis illumination.

In a preferred embodiment of the invention, a first, off-axis,illumination source is placed substantially at one focus of a reflectorwhich forms a minor part of a cylindrical elliptical surface. Theworkpiece is placed at the other focus of the reflector. The workpieceis oriented at an angle to the off-axis illumination such that, if theworkpiece were a specular reflector, light reflected from it would beoriented such that it would not be reflected back toward the mirror.Thus, the surface of the workpiece is not perpendicular to the center ofthe off-axis illumination.

In a preferred embodiment of the invention, a narrow rectangular planarstrip mirror is placed between the off-axis source and the reflector,such that the off-axis illumination reaching the workpiece is split intotwo non-joining parts, each part having a substantially equal angle ofillumination and separated by an angular wedge shaped gap. Imagecollecting optics is placed such that its axis just intercepts theextreme edge of the off-axis illumination.

In a preferred embodiment of the invention, the on-axis source is placedsubstantially at a virtual focus of the ellipse as reflected by theplanar strip mirror. Light from this source will be reflected from themirror and will illuminate the workpiece with on-axis illumination withrespect to the collecting optics. The two sources will provide aseamless illumination of the workpiece since light from the off-axissource will illuminate from angles which are not “covered” by the stripand the on-axis source will illuminate only from central angles thusfilling the gap between the parts of the off-axis illumination. Thissystem is seamless, self aligning and provides for off-axis and on-axisillumination, seemingly from a single source but which has separateadjustability for the off-axis and on-axis illumination respectively.

In a preferred embodiment of the invention, a diffuser is placed infront of each of the off-axis and on-axis sources such that each ofthese sources provides a different, broader effective source ofillumination. One aspect of some embodiments of the invention provides amethod of determining the effective width and effective position of theresulting sources of illumination.

Yet another aspect of some preferred embodiments of the inventionprovides for multi-spectral detection of light reflected from theworkpiece. In a preferred embodiment of the invention, R, G & B areseparately detected and a composite “gray level” reflection value isgenerated by weighting the measured intensities of the three detectedcolors. Preferably the weighting is determined to give an optimumcontrast between the metal and substrate for printed circuit boards (orfor other workpieces in which the elements being differentiated havedifferent colors) and is a function of color of the two materials, ofthe color of the illumination and possibly of the extent of theillumination. It should be understood that other methods of weightingsuch as filtering of the reflected light may be utilized. However, suchmethods are less precise and generally less efficient than weighting thesignals. In a preferred embodiment of the invention, the light ispreconditioned, as by filtering, to provide substantially independentdetection of different spectral segments by the detectors.

There is thus provided, in accordance with a preferred embodiment of theinvention, illuminator apparatus for illuminating a workpiece duringvisual testing thereof, the illuminator comprising:

-   -   a source of illumination that illuminates a portion of the        workpiece with on axis illumination having a first intensity and        off-axis illumination having a second intensity; and    -   an optical viewing system, that views said portion of the        workpiece and accepts light reflected from the workpiece over a        range of angular directions, said range of angular directions        defining said on-axis illumination,    -   wherein said on-axis and off-axis illumination have separately        adjustable intensities and appear to emanate from a contiguous        source.

There is further provided, in accordance with a preferred embodiment ofthe invention, illuminator apparatus for illuminating a workpiece duringvisual testing thereof, the illuminator comprising:

-   -   a source of illumination that illuminates a portion of the        workpiece with on-axis illumination centered at a first angular        direction and having a first intensity and with off-axis        illumination having a second intensity; and    -   an optical viewing system, that views said portion of the        workpiece and accepts light reflected from the workpiece over a        range of angular directions, centered at a second angular        direction, said range of angular directions defining said        on-axis illumination,    -   wherein said first intensity and second intensity are separately        adjustable and wherein the first angular direction is different        from the second angular direction.

Preferably, the off-axis and on-axis illumination together illuminatethe workpiece over a second range of angles, said second range of anglesbeing substantially centered at said first angular direction.

Preferably, the illumination illuminates the workpiece from anglesranging over substantially less than a total range of 180°, morepreferably less than 100°.

In accordance with a preferred embodiment of the invention, the sourceof illumination includes a first source of illumination that producesthe on-axis illumination and a second source of illumination thatproduces the off-axis illumination. Preferably, the illuminationproduced by the first source and the second source illuminate theworkpiece without any gap between the on-axis and off-axis illumination.Preferably, the apparatus includes a mirror from which the first sourceis reflected to illuminate the workpiece. Preferably, the mirror has anextent and the second source is situated behind the mirror such that themirror blocks illumination therefrom from illuminating the workpiece andwherein said off-axis illumination is comprised of illumination from thesecond source which passes the mirror outside its extent. Preferably,the mirror is mounted on a transparent substrate having an extentgreater than the extent of the mirror.

In a preferred embodiment of the invention, the first source and thesecond source are substantially optically equidistant from the mirror.

A preferred embodiment of the invention includes a concentrating mirrorthat receives illumination from the first and second sources andconcentrates the illumination onto the workpiece. Preferably, theconcentrating mirror comprises an elliptical mirror portion. Preferably,the first and second sources are optically situated substantially at afocus of the elliptical mirror.

In a preferred embodiment of the invention, the concentrating mirrorcomprises a base having the general shape of the mirror and a metal foiladhered to the base. Preferably, the metal foil is adhered to the baseby a vacuum applied to the metal foil.

In a preferred embodiment of the invention, the workpiece is situatedsubstantially at a second focus of the elliptical mirror.

In a preferred embodiment of the invention the first source and thesecond source are line sources. Preferably, the line source comprises aradiant line source and a diffuser through which light that illuminatesthe workpiece from said line source passes.

There is further provided, in accordance with a preferred embodiment ofthe invention, apparatus for visual inspection of a workpiececomprising:

-   -   illumination apparatus according to any of the preceding claims;        and    -   an optical sensor that receives light from the optical viewing        system and produces image signals in response thereto.

Preferably, the apparatus includes:

-   -   means for moving the workpiece relative to the illumination        apparatus such that the optical sensor produces image signals        representative of successive portions of the workpiece.

Preferably, the on-axis illumination illuminates the workpiece from arange of directions having an angular extent of between about 4° andabout 8°, more preferably about 6°.

In a preferred embodiment of the invention, the on-axis and off-axisillumination illuminate the workpiece from a range of directions havingan angular extent of between 30° and about 60°, more preferably betweenabout 39° and 45°.

In preferred embodiments of the invention, the workpiece comprises aprinted circuit board.

There is further provided, in accordance with a preferred embodiment ofthe invention, a mirror comprising:

-   -   a base having the general shape of the mirror; and    -   a metal foil forming a mirror surface adhered to the base by a        vacuum.

Preferably, the metal foil has a thickness between about 0.25 mm andabout 0.45 mm, more preferably between about 0.25 mm and about 0.35 mmand most preferably about 0.35 mm.

There is further provided, in accordance with a preferred embodiment ofthe invention, an illuminating system for illuminating a workpieceduring visual inspection thereof, comprising:

-   -   a linear source of radiation, comprising:        -   a line source;        -   a diffuser: situated on one side of the line source; and    -   a reflector having at least one focus situated on the other side        of the diffuser from the line source, wherein an effective        position of the linear source of radiation situated between the        line source and the diffuser is situated at the focus of the        reflector.

There is further provided, in accordance with a preferred embodiment ofthe invention, an illuminating system for illuminating a workpieceduring visual inspection thereof, comprising:

-   -   a light source emitting light over a continuous angle of        illumination toward the workpiece;    -   a blocking element that block light over a portion of the        continuous angle such that two portions of the illumination,        separated by a blocked angle illuminate the workpiece from the        source.

Preferably the apparatus comprises a concentrator that receives theunblocked portions of light and concentrates the illumination onto theworkpiece. Preferably, the source of light is placed at a focus of theconcentrator. Preferably, light from the source of light is concentratedat a focus of the concentrator. Preferably, the concentrator is amirror.

In a preferred embodiment of the invention the concentrator is anelliptical mirror and wherein the source of light is situated at onefocus of the elliptical mirror and the light emitted by the source isconcentrated at a second focus of the elliptical mirror.

Preferably, the position of the blocking element is adjustable, suchthat the angular extent of the two sections of illumination isadjustable.

Preferably, the blocking element supplies illumination to the workpieceover the blocked portion of the continuous angle.

In a preferred embodiment of the invention, the blocking elementcomprises a planar strip mirror having a mirrored surface facing awayfrom the light source. Preferably, the apparatus includes an additionalsource of illumination that supplies illumination to the mirror fromwhich it is reflected in generally the same direction as theillumination from the light source. Preferably, the light source are atthe same effective position when viewed from the mirror side of thestrip mirror.

In a preferred embodiment of the invention one or more of the intensity,polarization and wavelength of at least one of the light source and theadditional source are separately adjustable.

There is further provided, in accordance with a preferred embodiment ofthe invention, an automated optical inspection system for inspectingsubstantially flat workpieces, comprising:

-   -   illuminating apparatus as described above;    -   an imager that images a workpiece illuminated by the        illuminator; and    -   an image analyzer that analyzes the image and determines the        existence of defects in the workpiece.

The invention will be more clearly understood by reference to thefollowing description of preferred embodiments thereof read inconjunction with the accompanying figures. Identical structures,elements or parts that appear in more than one of the figures arelabeled with the same numeral in all the figures in which they appear.Analogous structures, elements or parts that appear in more that one ofthe figures are labeled with an analogous series of numerals in thefigures in which they appear.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a simplified cross sectional view of an illuminator, inaccordance with a preferred embodiment of the invention;

FIG. 2 is a schematic cross-sectional view of an illuminator, inaccordance with a preferred embodiment of the invention;

FIG. 3 is a schematic cross section of a diffused source, in accordancewith a preferred embodiment of the invention;

FIG. 4 illustrates ray tracing of light rays used to determine a virtuallight source corresponding to the source of FIG. 3, in accordance with apreferred embodiment of the invention;

FIG. 5 schematically illustrates a method of illuminating andconstructing a long linear source of illumination; in accordance with apreferred embodiment of the invention; and

FIG. 6 is a block diagram illustration of a printed circuit boardinspection machine including the illumination system of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a perspective view of a simplified schematic of anilluminator 10, in accordance with a preferred embodiment of the presentinvention. Illuminator 10 comprises a cylindrical reflector or mirror 12having a substantially elliptical shape and a limited extent with anoff-axis substantially uniform linear source of illumination 14 placedat one focus thereof and a workpiece or other substrate 16, to beoptically examined placed at another focus thereof. A front surfacestrip mirror 18 is placed between off-axis source 14 and mirror 12, withthe mirrored surface facing away from off-axis source 14. The width ofstrip mirror 18 is such that it allows light from off-axis source 14 tobe concentrated by mirror 12 and to reach substrate 16 from two angularsectors each having a width β each as shown, separated by a wedge shapedregion of angular extent γ. It should be noted that while strip mirror18 is shown as being mounted on a substrate 20 which has the same widthas strip mirror 18, substrate 20 may be wider than the strip mirror itsupports (as shown in FIG. 2). It is readily appreciated that theposition and arrangement of mirror 12 may be adjusted to adjust therelative angular extents of each of sectors β.

A narrow substrate 20, as shown, may be formed completely of metal. Sucha structure introduces fewer aberrations into the off-axis illuminationthan when a wide glass support is used. The choice between the twostructures is based, at least in part by the length of the strip mirrorand the stability requirements of the system. In a preferred embodimentof the invention, the dimension (β+γ/2) may vary from about 15° to 30°,and most preferably is about 19.5° to 22.5°. In a preferred embodimentof the invention γ may vary from about 4° to 8° and most preferably isabout 6°.

A second, on-axis linear illumination source 22, similar to off-axissource 14, is placed, in accordance with a preferred embodiment of theinvention, substantially at the reflection of the focus of theelliptical reflector as reflected by strip mirror 18. Thus, illuminationfrom on-axis source 22, reflected by strip mirror 18 illuminates theother focus of the elliptical reflector within the same angular extentas that which mirror 18 blocks the illumination from source 14, namelythe angle γ.

To more clearly understand how a preferred embodiment of theillumination system of FIG. 1 provides substantially seamlessillumination, assume that only the off-axis source 14 is on. Light raysfrom the off-axis source 14 that are incident on the back surface ofstrip mirror 18 are “shadowed out” from the front side of the stripmirror. Strip mirror blocks these rays and they are thus are notconcentrated by mirror 18 onto workpiece 16. Light rays from off-axissource 14 that are not incident on the strip mirror pass to the rightand left sides of the strip mirror 18.

Assume the on-axis source 22 is turned on. Light rays from the on-axissource 22, which is adjacent to the front side of strip mirror 18, areincident on the front surface of strip mirror 22, and are reflectedtherefrom to reflector 12. Light rays from on-axis source 22 that arenot incident on the strip mirror pass to the back side of the stripmirror.

Because the on-axis source is at substantially the same point opticallyas the off-axis source, light rays from the on-axis illumination sourcethat are reflected back to the front side of the strip mirror appear tocome from the same point as does the off-axis illumination source. Ifthe angular radiance of the two sources is the same, these reflectedlight rays from the on-axis source replace those light rays from theoff-axis source in the gap shadowed out by the strip mirror. The stripmirror “seamlessly” combines light from the off-axis illumination sourceand the on-axis illumination source so that all the light on the frontside of the strip mirror appears to be radiated by a single illuminationsource. The alignment of such a system is thus seen to be not onlysimple and straightforward but also much less critical than that of mostprior art systems. Furthermore, the optical components may be lessprecise than in prior art systems. Finally, the number of components isgenerally reduced and the components are more simply (and inexpensively)formed and assembled.

In a preferred embodiment of the invention, sources 14 and 22 areprovided by illuminating a fiber bundle and then spreading the fibersinto a line source. This provides a uniform linear illumination source.

While the angular extent of the left and right off-axis illumination isadjustable by positioning of strip mirror 18, it has been found topreferably be substantially the same. It may be noted that when theangular extent of each of the left and right off-axis illuminations isthe same, the portion of mirror 12 which is illuminated by each of theleft and right off-axis illuminations is not the same.

It may also be noted that the illumination intensity of the on-axis andoff-axis sources respectively may be individually adjusted. This allowsfor attaining an even illumination across the entire angle ofillumination (2β+γ), or for making either the intensity of either theon-axis illumination or the off-axis illumination stronger relative tothe other. Alternatively or additionally the polarization or wavelengthof the off-axis and/or the on-axis sources may be varied.

In a preferred embodiment of the invention mirror 12 is formed of analuminum foil having a nominal thickness of preferably between about0.25 mm and 0.35 mm, most preferably about 0.35 mm±about 5 micron. Thesevalues are chosen to give reasonable precision and stability to themirror. In a preferred embodiment of the invention, mirror 12 is mountedon a machined part 13 to which it is attached by vacuum (with areflective surface facing away from the machined part) such that therequired optical surface finish is provided by the aluminum foil and theshape is provided by the machined part.

Collecting optics 24 is provided to image the illuminated line on theworkpiece onto a plurality of detectors, (such as CCD elements)preferably formed into one or more line detectors 25, 26 and 27 (shownas points on FIG. 1 since they are viewed end on). The axis of collectoroptics 24 is oriented at an angle with respect to the workpiece suchthat it would intercept the central ray of the on-axis illumination thatwould be reflected from the workpiece, were the workpiece to be aspecular reflector. The aperture of the collector optics is such that itwould be overfilled by the on-axis radiation from such a specularsurface. In a preferred embodiment of the invention, as will bedescribed in greater detail below, a series of axially spaced lenses orlens systems forming an optical array 28 (FIG. 6) is provided, each ofwhich lens systems images one of a plurality of preferably overlappingportions of the illuminated portion of the workpiece onto CCD lines25-27.

Each of the CCD line detectors may comprise a single axial line of suchdetector elements or a strip a few elements wide. Such a strippreferably images a strip segment of the workpiece which is about 0.5min wide. This width and the spacing of the CCD elements in a linedetector may vary depending on the desired measurement resolution of thesystem. It should be understood that the effective optical length andspacing of the CCD elements in the direction of motion of the workpiecedepends on the sampling rate of the signal from the detectors and thevelocity of the workpiece as it passed through the illuminator.

In a preferred embodiment of the invention, multi-spectral detection oflight reflected from the workpiece is performed, and each of lines 25-27is sensitive to a different, preferably substantially non-overlapping,portion of the spectrum. In a preferred embodiment of the invention, R G& B are separately detected and a composite “gray level” reflectionvalue is generated by weighting the measured intensities of the threedetected colors. Preferably the weighting is determined to give anoptimum contrast between the metal and bare substrate, and is a functionof color of the two materials, of the color of the illumination andpossibly of the extent of the illumination. It should be understood thatother methods of weighting such as filtering of the reflected light byfilters (not shown) may be utilized. However, such methods are lessprecise and generally less efficient than weighting the signals. In apreferred embodiment of the invention, the light is preconditioned, asby filtering, to provide substantially independent detection ofdifferent spectral segments by the detectors. This may be desirable whenthe detector elements are sensitive to overlapping spectral segments.

In preferred embodiments of the invention, one or more improvementsand/or variants on the basic system as described with respect to FIG. 1may be employed. These improvements are described together with respectto FIGS. 2 and 3. However, it should be understood that preferredembodiments of the invention may have one or more, or none of theembellishments of FIGS. 2 and 3.

In accordance with a preferred embodiment of the invention, each oflinear sources 14 and 22 (FIG. 1) comprises, now as shown in illuminator110 (FIG. 2) a wide strip of source illumination, 114 and 122. Forsimplicity of the drawing, only on-axis source 122 in FIG. 2 is shown asan extended source. As is evident from FIG. 2, on-axis-source 122illuminates a region of a workpiece 116 having a given width rather thanthe line illumination shown in FIG. 1.

In general, the illumination system shown has aberrations. Since theacceptance angle of the off-axis illumination is larger than that of theon-axis illumination and since it depends on a larger portion of mirror112 for its focusing effect, it can be expected to be more subject tovariations in intensity near the edges of the field. Therefore, in apreferred embodiment of the invention, source 114 is wider than source122. Preferably, only that portion of the workpiece for which uniformillumination by source 122 is provided, is used in the analysis of theworkpiece.

FIG. 2 also shows the effect of a finite thickness and extent ofsubstrate 120 on which strip mirror 118 is mounted. The net effect ofthe substrate 120 is to cause a slight deflection in the position ofsource 114 as well as a difference in aberrations for the on-axis andoff-axis illumination. This causes a small but usually unobjectionable,and sometimes desirable, smoothing of the joining of the off-axis andon-axis illumination. Preferably source 114 will be slightly broadenedto prevent loss of illuminance uniformity due to the aberrations.

The uniformity of illumination is controlled by the relative radiance ofon-axis illumination and off-axis illumination as observed at each pointin the strip region being illuminated 123. When the radiance (light fluxper unit area and unit solid angle) of light radiated by the on-axisillumination source is controlled to be substantially equal to theradiance of light radiated by the off-axis illumination source, eachpoint in the illuminated strip region 123 is illuminated bysubstantially equal illumination intensity for incident angles of lightrays between angles ±(β+β/2). Under these conditions, in the range ofincident angles between ±(β+γ/2), an illuminator in accordance with apreferred embodiment of the present invention provides substantiallyuniform illumination. For PCB inspection, the total half angle ±(β+γ/2)is preferably limited to less than 30°, and more preferably to about22.5°, so as to obtain an optimum contrast and signal to noise ratio.

In an illuminator in accordance with a preferred embodiment of thepresent invention, strip mirror 118 is a narrow reflecting lamination ordeposit on a planar surface of a relatively rectangular substrate 120,such as a plate of glass. Strip mirror 118 and off-axis light source 114are preferably positioned with respect to the substrate 120, and thesubstrate 120 is preferably sufficiently wide, so that all the lightfrom the off-axis illumination source that is concentrated onto a stripof a workpiece passes through the glass of substrate 120.

The structure of a preferred illumination source for use with apreferred embodiment of the present invention shown in FIG. 2, is nowshown in FIG. 3. Linear light sources 114 and 122 used for on-axis andoff-axis illumination preferably comprise ends of optical fibers 136that are fanned out from at least one fiber bundle 138 so that the endsare coplanar and lie in a dense linear array having a shape of a longnarrow rectangle. The bundle of end or ends of the fiber bundle isoptically coupled to a lamp or a plurality of lamps 150 (FIG. 5). Lightfrom the lamps is piped through the length of the optical fibers to theends of the optical fibers in the linear array, from which ends thelight is radiated.

FIG. 5 shows, schematically, the construction of light source 114, inaccordance with a preferred embodiment of the invention. In accordancewith a preferred embodiment of the invention, a very long strip of theworkpiece is to be illuminated, preferably of the order of 660 mm. Inorder to provide such a long strip of illumination and to assure uniformlight intensity along the entire strip, a plurality of light sources,such as high intensity lamps 150 (suitably 250 W HLX reflector typequartz halogen lamps provided by Osram Corporation) are utilized. Eachof lamps 150 illuminates a bundle 38 of optical fibers. As shown in FIG.5 four lamps 150 and four bundles 138A-138D are preferably used foroff-axis source 114. At one end of bundles 138A-138D the fibers areformed into three sets of fiber ends 136A-136C. Each set 136A-136Ccomprises four layers 156A-156D, each layer being formed from fibersfrom one of bundles 138A-138D. Thus, each of sets 136A-136C comprisesfour layers of light fiber ends each layer being illuminated by adifferent light source. The total light supplied to each of the sets isequal and this equality does not depend on the balance between the lightsources.

In a preferred embodiment of the invention, the four layer sets136A-136C are laid end to end to form a uniform linear light source fourfiber layers thick (about 0.56 mm) and over 660 mm long. This lightsource has been and will continue to be referred herein to as fiber ends136.

In an exemplary embodiment of the invention, each fiber bundle includes17,025 fibers, each having an 80 micrometer diameter. Suitable opticalfiber bundles are available from the Schott Company of Germany.

Light source 122 is produced in a manner similar to that employed in theproduction of source 114, except that only two fiber bundles andassociated lamps are employed. Thus each of the three sections 136 isconstructed of only two layers (one from each bundle) and the lightsource is only 0.28 mm thick.

In the preferred embodiment of the light source according to theinvention shown in FIG. 3, light radiated from the fiber ends 136 isincident on a diffuser 140 which is parallel to the array of fiber ends136 and substantially perpendicular to central light rays in asubstantially wedge shaped beam of light radiated by the array of fiberends.

The light incident on the diffuser produces an illuminated band on thediffuser that is defined by a width, and on passing through the diffuserthe light appears to emanate from a magnified virtual linear lightsource behind the diffuser.

The present inventors have found, using a ray tracing technique asillustrated in FIG. 4, that, in fact, the apparent source of theillumination is neither diffuser surface 140 nor the fiber ends 136.Rather, the light actually appears to be emanating from a virtual lightsource 142 located behind actual source. However, the present inventorshave surprisingly found that the position and width of an effectivelight source is different from that of both fiber ends 136 and thevirtual source 142. Rather it is defined by a long four sided strip 144which is referred to herein as a “virtual effective source.” This sourceis situated between the virtual linear light source 142 and the diffuser140. The size and location of the virtual source is determined mainly bythe width 151 of the illuminated band on the diffuser and the aperture152 of the elliptically cylindrical mirror, for the off-axis source andthe strip mirror for the on-axis source. It is also affected by thediffusing angle δ of the diffuser. While the workpiece is actuallyilluminated by light outside of the virtual light source, this lightsupplies uneven illumination since it is vignetted because not all of itpasses through the aperture of the system.

The virtual effective light source is thus that source which providessubstantially unvignetted illumination of the workpiece, and for thepurposes of illuminating the workpiece, provides desirable lighting.Preferably, the one focus of reflector 112 is optically coincident withthe virtual source 144 (after correction for the effect of substrate 120for the off-axis source), as a result of which the uniformity ofintensity of concentrated illumination of the strip on the workpieceacross the width of the strip is improved. Light emerging from regionsfar from the focus of the reflector and especially from outside thevirtual effective source are not in focus at the workpiece surface.

Aberrations in optical elements of an illuminator constructed accordingto the teachings herein generally distort and blur the image of theeffective light source. This effect further limits the region of theworkpiece which is uniformly illuminated, i.e. the region that receiveslight from all parts of the illuminating aperture within the angle±(β+γ/2). The effect of the aberrations is to partially eclipse parts ofthe illuminating aperture. Generally, off-axis illumination is moresusceptible to distortion than on-axis illumination because largeroptical apertures and incidence angles are used to focus off-axisillumination than on-axis illumination. For example, in prior artilluminators two separate optical systems are generally used foroff-axis illumination while only one optical system is used to focuson-axis illumination.

In order to accommodate aberrations and compensate for their effects, inaccordance with a preferred embodiment of the present invention, thewidths of the on- and off-axis sources are different so that strips thatare illuminated by on- and off-axis illumination on a workpiece beinginspected are wider than would be needed in the absence of aberrations.The required width of each respective source is determined by tracingrays from the required region on the workpiece back to the source planethrough all optics, while accounting for production tolerances. Therequired source width should encompass all such back traced rays. Arelatively uniform illuminated strip within the “widened” on- andoff-axis strips is located and used for inspection of the workpiece. Thepositions of the fiber ends 36 and their distance from the diffuser ischosen to provide an effective light source having an extent whichprovides the desired width of illumination of the surface of theworkpiece by the on-axis and off-axis illumination.

As indicated above the off-axis illumination is more susceptible todistortion than the on-axis illumination. In accordance with a preferredembodiment of the present invention, the width of the off-axisillumination source is preferably larger than the width of the on-axissource so that the off-axis illumination illuminates a wider strip onthe workpiece than the higher quality on-axis illumination. The area inwhich the on-axis and off-axis illumination overlaps is scanned tolocate an optimum, relatively straight, distortion free illuminatedstrip appropriately illuminated by on- and off-axis illumination. Thisoptimum illuminated strip (generally narrower than the strip illuminatedby the on-axis illumination) is the strip on which the collecting opticsof an imaging system used to inspect the workpiece are focused and whichis imaged on the detectors.

This scanning is performed in the absence of a workpiece using amicroscope which images light radiated by the on- and off-axisillumination sources in the region where both sources are concentratedin order to illuminate a strip on a workpiece. The microscope iscontrolled to travel the length of the region where the on- and off-axisillumination are concentrated and the position of the microscope isaccurately monitored as it moves. Light from the on- and off-axissources imaged by the microscope is analyzed to determine the size andlocation of an optimum illuminated strip on which to focus thecollecting optics of an imaging system.

From the above discussion it is seen that an illuminator, in accordancewith a preferred embodiment of the present invention, comprises areduced number of optical elements and/or elements which are easier tofabricate and/or are easier to assemble into the illuminator, incomparison to most prior art illuminators. An illuminator, in accordancewith a preferred embodiment of the present invention, providesseparately adjustable on-axis illumination and substantially symmetricoff-axis illumination for all points in an illuminated strip using twolight sources and a single focusing system. Prior art illuminators whichprovide separate adjustment of the on-axis and off-axis illuminationgenerally use at least three light sources, each with its own focusingsystem. As a result, an illuminator in accordance with a preferredembodiment of the present invention is comparatively simple, andrequires machining tolerances for components that are generally not assevere as the tolerances required for components used in prior artilluminators.

An immediate benefit of these advantages is not only the potentiallyreduced manufacturing costs. An illuminator in accordance with apreferred embodiment of the present invention is capable of providingillumination for strips on a workpiece that are much longer than thosethat can be effectively illuminated by prior art illuminators. Thisenables large workpieces that according to prior art required at leasttwo passes through an inspection system to be inspected, completelyinspected, in accordance with a preferred embodiment of the presentinvention, in a single pass through an inspection system. The inventorsof the present invention have found that an illuminator in accordancewith a preferred, embodiment of the present invention can provideeffective illumination for a strip on a workpiece having a lengthgreater than 60 cm.

A block diagram illustration of a system for the inspection of printedcircuit boards (PCBs) preferably implementing an illuminator constructedin accordance with a preferred embodiment of the illuminator is shown inFIG. 6.

An inspection system 200, shown in FIG. 6, comprises an optical array260 comprising a plurality of illuminators 10 each with their respectivecollecting optics 24 (FIG. 1).

A conveyor 262 is arranged to transport workpieces, such as PCBs 264, tobe inspected past optical array 260 in a transport direction indicatedby an arrow 266. A suitable conveyor for use in a preferred embodimentis described in U.S. patent application Ser. No. 09/010,582 and EPPatent application 97300521.8, the disclosures of which is incorporatedherein by reference. Inputs comprising a sequence of optical images ofstrips of the PCB 264 scanned as it passes under optical array 260 aretransferred to a computer 268.

In a preferred embodiment of the invention, optical array 260 iscomprised of three illuminator units 10, each unit including separatecollecting optics 24 (FIG. 1). Each of CCD lines 25-27 comprises adiscontinuous line of detectors. The separate collector optics image alinear segment of the workpiece surface onto one of the sections of linedetectors 25-27. Preferably the respective fields of view of thecollector optics overlap somewhat so that the sections may be referencedto each other or more preferably joined into a complete image of theworkpiece. The combined fields of view of the collector opticspreferably extend across substantially the entire width of conveyor 260.

Furthermore, it will be noted that in a preferred embodiment of theinvention, the three CCD lines 25, 26 and 27 each represent Red, Greenand Blue color spectrums respectively, and are offset such that theyimage slightly offset lines on the illuminated portion of the workpiece.Thus as the workpiece moves beneath the illuminating source, the imagesacquired by the lines of detectors will be offset by the spacing of thelines of detectors. In accordance with preferred embodiments of theinvention, these multi-color images, as well as the relative offsets ofthe collector optics, are realigned by computer processing, which takesinto account both the spacing of lines 25, 26 and 27 and the velocity ofPCB 264.

Image reconstruction and subsequent processing is performed by computer268. Optical Correction Circuitry 270 is provided to reconstruct animage of the inspected PCB and compensate for optical aberrations inacquired images. In a process, typically performed periodically, aninspection test is conducted on system 200 using a test pattern of knownconfiguration. The results from scanning the test pattern are processedto determine optical aberrations, preferably including the respectivespatial alignment of images acquired from optical arrays 260, imageoverlap, differences in accumulation times and the like. Necessarycorrections are computed off-line and the results are stored in aMapping Function Generator. 272 During on-line inspection, opticalcorrection data stored in the Mapping Function Generator 272 is suppliedOptical Correction Circuitry 270 and used to effect optical correctionof the images acquired from optical array 260 and to reconstruct anoptically corrected image of the inspected PCB.

Optical Correction Circuitry 270 provides a corrected sensor arrayoutput to Image Pre-Processing Circuitry 274. Image Pre-ProcessingCircuitry 274 preferably provides a segmentation output indicationdividing all areas on the image represented by the corrected sensorarray output into categories. For example, in the case of PCBs, everylocation on the image represented by the corrected sensor array outputis identified by the segmentation output indication as being eitherlaminate or conductor. Additionally, Image Pre-Processing circuitry 274may provide a separate segmentation information based on predeterminedgray-scale thresholds in certain color spectra, for example therebyadditionally segmenting regions suspected of being oxidized ornon-oxidized copper.

A segmentation output indication from Image Pre-Processing Circuitry 274is supplied to Image Processing Circuitry 276. Image ProcessingCircuitry 276 provides an image processing output which identifiesvarious features of the image on the PCB and their respective locations.In the case of printed circuit boards, the features are typically pads,conductor junctions and conductive paths, vias and the like, as well asindications of their width, shorts and discontinuities therein. ImageProcessing Circuitry is preferably a morphology based system, but mayalternatively be bit map, net list, design rule or contour based, orbased on any other suitable input or combination of the above.

An image processing output of Image Processing Circuitry 276 is suppliedto Feature List Registration Circuitry 278, which maps the coordinatesystem of the output of Image Processing circuitry 276 onto a referencecoordinate system. This registration mapping may be tuned dynamicallyusing dynamic registration methods known in the art in accordance withinformation supplied by a Feature Reference Source 280, such as, forexample, that described in U.S. Pat. No. 5,495,535.

An output of Feature List Registration circuitry 278 and an output ofFeature Reference Source 280 are supplied to Feature ComparisonCircuitry 282, which circuitry compares the now registered map of outputof Image Processing Circuitry with a reference stored in FeatureReference Source 280 and provides an output indication of defects. Suchdefects, in the context of printed circuit board inspection, maytypically include absence of required features, the existence ofunnecessary features, shorts or discontinuities in connectors,incorrectly shaped features, oxidation and the like defects. An outputof the Feature Comparison Circuitry 282 is supplied to a Defect OutputGenerator Circuitry 284 that prepares a report of defects found on thePCB, which may then be used to assist in manually inspection ofdefective regions on the PCB. As appropriate, defective PCBs may berepaired or discarded.

While certain features of inspection system 200 have been indicated asbeing performed by hardware, it should be understood that such featuresmay be performed by software, firmware or combinations of the softwareand firmware. Furthermore, the functions may be performed utilizingcombinations of software, firmware and dedicated hardware.

While the invention has been described with reference to certainpreferred embodiments, various modifications will be readily apparent toand may be readily accomplished be persons skilled in the art withoutdeparting from the spirit and the scope of the above teachings. Inparticular, certain embodiments of the invention may not have all of thefeatures of the above described embodiments and further some embodimentsof the invention may combine features described above with reference todifferent embodiments of the invention. Therefore, it is understood thatthe invention may be practiced other than as specifically describedherein without departing from the scope of the following claims:

1. Illuminator apparatus for illuminating a workpiece during visualtesting thereof, the illuminator comprising: a first illumination sourcearranged to direct light onto a portion of the workpiece from a wedgeshaped sector of directions, centered at an illumination axis; a secondillumination source arranged to direct light onto said portion from twoangular sectors, said two angular sectors being separated by said wedgeshaped sector; an optical viewing system, arranged to view said portionof the workpiece and accept light reflected from the workpiece over arange of angular directions, wherein said first and second illuminationsources have separately adjustable intensities and said secondillumination source comprises a same source for providing saidillumination for said two angular sectors.
 2. Apparatus according toclaim 1 wherein the wedge shaped sector and the two angular sectors forma contiguous illumination sector.
 3. Apparatus according to claim 1wherein the range of angles encompassed by the illumination from thewedge shaped sector and the two angular sectors is substantially lessthan a total range of 180°.
 4. Apparatus according to claim 3 whereinthe illumination is arranged to illuminate the workpiece from anglesranging over substantially less than a total range of 100°.
 5. Apparatusaccording to claim 1 wherein the optical viewing system views theportion over a range of viewing angles, centered at a viewing axis, theviewing axis being at an angle offset from said illumination axis. 6.Apparatus according to claim 1 wherein the range of angles encompassedby the illumination from the wedge shaped sector and the two angularsectors is centered at the illumination axis.
 7. Apparatus according toclaim 1 including a mirror from which the first source is reflected toilluminate the workpiece.
 8. Apparatus according to claim 7 wherein themirror has an extent and the second source is situated behind the mirrorsuch that the mirror blocks illumination therefrom from illuminating theworkpiece and wherein said off-axis illumination is comprised ofillumination from the second source which passes the mirror outside itsextent.
 9. Apparatus according to claim 7 wherein the mirror is mountedon a transparent substrate having an extent greater than the extent ofthe mirror.
 10. Apparatus according to claim 7 wherein the first sourceand the second source are substantially optically equidistant from theminor.
 11. Apparatus according to claim 7 and including a concentratingmirror arranged to receive illumination from the first and secondsources and concentrate the illumination onto the workpiece. 12.Apparatus according to claim 11 wherein the concentrating mirrorcomprises an elliptical mirror portion.
 13. Apparatus according to claim12 wherein the first and second sources are optically situatedsubstantially at a focus of the elliptical mirror.
 14. Apparatusaccording to claim 11 wherein the concentrating mirror comprises a basehaving the general shape of the mirror and a metal foil adhered to thebase.
 15. Apparatus according to claim 14 wherein the metal foil isadhered to the base by a vacuum applied to the metal foil.
 16. Apparatusaccording to claim 11 wherein the workpiece is situated substantially ata second focus of the elliptical minor.
 17. Apparatus according to claim1 wherein the first source and the second source are line sources. 18.Apparatus according to claim 17 wherein at least one of the first andsecond line sources is comprised of a plurality of sources that extendalong a line.
 19. Apparatus according to claim 17 wherein the linesource comprises a at least one radiant source and a diffuser throughwhich light that illuminates the workpiece from said source passes. 20.Apparatus for visual inspection of a workpiece comprising: illuminationapparatus according to claim 1; and an optical sensor so situated thatit receives light from the optical viewing system and operative produceimage signals in response thereto.
 21. Apparatus according to claim 20wherein the illumination apparatus include: a transporter operative tomove the workpiece relative to the illumination apparatus such that theoptical sensor produces image signals representative of successiveportions of the workpiece.
 22. Apparatus according to claim 1 whereinthe on-axis illumination illuminates the workpiece from a range ofdirections having an angular extent of between about 4° and about 8°.23. Apparatus according to claim 22 wherein the angular extent of theon-axis illumination is about 6°.
 24. Apparatus according to claim 1wherein the on-axis and off-axis illumination illuminate the workpiecefrom a range of directions having an angular extent of between 30° andabout 60°.
 25. Apparatus according to claim 24 wherein the angularextent of the on-axis and off-axis illumination is between about 39° and45°.
 26. Apparatus according to claim 1 wherein the workpiece comprisesa printed circuit board.